On April 2, 1917, less than one month after beginning his second term in office, President Woodrow Wilson asked Congress to declare war on Germany to keep the world “safe for democracy.” Congress obliged.
The war didn't immediately affect the national pastime. At first the American public rallied around the flag, and enlistment in the Army was robust. The Cubs did their part too. They booked a demonstration at Wrigley by Canadian Highlanders to encourage recruiting. Recruiting booths were set up around the ballpark to facilitate the process.
The following season was a different story. Gruesome stories from the war started coming home, and people no longer wanted to enlist, so it became compulsory. Even though the Cubs were heading toward the NL Pennant, attendance hit an all-time low. That may have had something to do with the military's presence in the ballpark. By then Wrigley Field (still known as Cubs Park) had become a dangerous place for "slackers" (men who hadn't yet registered for military service).
They weren't just checking tickets at the gate, federal agents were standing by to check registration cards to make sure there were no draft dodgers attending the games. Anyone of draft age who didn't have a card was held for investigation. Scores of fans were led directly to military bases and inducted into the service.
It's funny how much that'll put a crimp into your box office receipts.